This invention relates to the synthesis of polymers from polyfunctional monomers, and more particularly to the synthesis of branched polycarbosilanes, polycarbosiloxanes, polycarbosilazenes and copolymers thereof.
Silicon-containing polymers, such as polycarbosilanes, polycarbosiloxanes, polycarbosilazenes and copolymers thereof, are typically synthesized from difunctional monomers to produce linear polymers, or from a combination of difunctional and polyfunctional monomers to prepare thermoset resins. Thermoplastic and thermoset polycarbosilanes, polycarbosiloxanes, polycarbosilazenes and copolymers thereof are used in a variety of applications. Liquid polycarbosilanes, polycarbosiloxanes, polycarbosilazenes and copolymers thereof are used as adhesives, lubricants, protective coatings, coolants, mold-release agents, dielectric fluids, heat transfer fluids, wetting agents, water-repellents, polishes, etc. Resinous polycarbosilanes, polycarbosiloxanes, polycarbosilazenes and copolymers thereof are used in coatings, molding compounds, sealants, room-temperature curing cements, modifiers for alkyd resins, etc. Elastomeric polycarbosilanes, polycarbosiloxanes, polycarbosilazenes and copolymers thereof are used for encapsulation of electronic parts, gaskets, surgical membranes, etc. However, there is a recognized need for branched, and more particularly for highly branched polycarbosilanes, polycarbosiloxanes, polycarbosilazenes and copolymers thereof. It is know that chemically similar polymers having different molecular architectures can exhibit different properties and advantages. For example, polymer-coating compositions comprising a highly branched polymer have a lower viscosity and better shear-thinning properties for coating applications than similar compositions containing a chemically similar linear polymer having the same molecular weight and same concentration.
One method of synthesizing branched polymers is to use polyfunctional monomers (i.e., monomers having three or more functional groups) during polymerization. However, this method may result in the production of gelled or thermoset cross-linked materials that do not exhibit good processability characteristics, and which are insoluble.
It has been suggested that dendrimers can be employed in certain applications to achieve improved properties, such as thermoplastic processing characteristics, lower viscosity, and improved rheology, as compared with linear polymers having similar chemistry and molecular weight. However, dendrimers are almost monodisperse (typically having a polydispersity of less than about 1.02), highly defined molecules that are prepared by a series of controlled stepwise growth reactions which generally involve protect-deprotect strategies and purification procedures at the conclusion of each step. As a consequence, synthesis of dendrimers is a tedious and expensive process that places a practical limitation on their applicability.
In contrast to dendrimers, hyperbranched polymers can be prepared in a one-step, one-pot procedure. This facilitates the synthesis of large quantities of materials, at high yields, and at a relatively low cost. Although the properties of hyperbranched polymers are different from those of dendrimers due to imperfect branching and larger polydispersities, hyperbranched polymers exhibit a degree of branching intermediate between that of linear polymers and dendrimers, and, therefore, exhibit thermoplastic processing and rheological properties that are comparable, or for some applications superior, to those of dendrimers. Accordingly, hyperbranched polymers have been perceived as being useful in certain applications as a lower cost alternative to dendrimers.
Heretofore, hyperbranched polymers, including hyperbranched polycarbosilanes, polycarbosiloxanes, polycarbosilazenes and copolymers thereof, have been prepared using a monomer having at least one functionality of one type (A), and at least two functionalities of another type (B), wherein functionalities of the same type are not reactive with each other, and functionalities of the first type are reactive with functionalities of the second type to form hyperbranched polymers via condensation or addition reactions. The monomers employed during synthesis of conventional hyperbranched polymers are generally designated as Ax By monomers, wherein A represents a functional group of a first type that does not react with itself, B represents a functional group of a second type that does not react with itself but reacts with the functional groups of the first type, x is at least 1, and y is at least 2. However, there are relatively few commercially available Ax By monomers, and synthesis of such monomers is generally more difficult than synthesis of monomers having a single type of functionality. As a result, the variety of hyperbranched polymers that can be synthesized from Ax By monomers is limited, and although they are generally less expensive than dendrimers they are often too expensive for many applications.
This invention provides hyperbranched polycarbosilanes, polycarbosiloxanes, polycarbosilazenes and copolymers thereof prepared from difunctional and/or polyfunctional monomers having functional groups of one type (A) without any other functional groups that react significantly during the polymerization process, and difunctional and/or polyfunctional monomers having functional groups of another type (B) that react with a functional group of the first type (A) without any other functional groups that react significantly during the polymerization process. More specifically, the hyperbranched copolymers of this invention are prepared by a process in which a monomer having the form Ax is reacted with another monomer of the form By, where A is a functional group that does not react with itself during the polymerization process, B is a functional group that does not react with itself during the polymerization process but participates in an addition or condensation reaction with a first functional group (A) to form a hyperbranched copolymer, x and y are integers which each have a value of at least 2, and at least one of x and y is an integer having a value of at least 3.
The process of this invention enables synthesis of hyperbranched polycarbosilanes, polycarbosiloxanes, polycarbosilazenes and copolymers thereof from comonomers, each of which has a single type of functional group, thereby providing greater flexibility in the preparation of a larger variety of different hyperbranched polycarbosilanes, polycarbosiloxanes, polycarbosilazenes and copolymers thereof. Because Ax and By type monomers are easier to synthesize than Ax By type monomers, the process of this invention also enables hyperbranched polycarbosilanes, polycarbosiloxanes, polycarbosilazenes and copolymers thereof to be prepared at a lower cost than with conventional synthesis processes.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification and claims.
This invention is directed to hyperbranched polycarbosilanes, polycarbosiloxanes, polycarbosilazenes and copolymers thereof prepared by reacting at least two different monomers, each of which will not react with itself, but will react with the other monomer, wherein at least one of the monomers includes at least three functional groups, and the other monomer is at least difunctional. It should be understood that the Ax and By monomers may contain other groups that are potentially reactive in other ways. Accordingly, it will be understood that an Ax monomer does not include any functional groups, other than the A-functional groups, that will react appreciably or significantly during polymerization reaction. Similarly, the By monomers will not contain any functional groups, other than the B-functional groups, that will react appreciably or significantly, during polymerization reaction. This requirement does not exclude the possibility of having functional groups that are potentially reactive under conditions other than those in which the polymerization is conducted, and does not exclude monomers having groups that are insignificantly reactive under polymerization conditions, i.e., other functional groups that are not reactive to an extent that prevents formation of a desired hyperbranched polymer. This process may be represented by the following example when x=2 and y=3:
a) if b/a greater than 1: 
b) if b/a less than 1: 
wherein xe2x80x9caxe2x80x9d represents the number of A2 molecules and xe2x80x9cbxe2x80x9d represents the number of B3 molecules.
Similar equations can also be written for other corresponding examples in which the parameters x and y may have other values.
For example, in accordance with this invention, a hyperbranched polycarbosilane is synthesized by a hydrosilation reaction of compounds having two or more vinyl, allyl or other homologous functional groups with a dihydrido or polyhydridosilane, wherein at least one of the monomers includes at least three functional groups. An example of such (A2+B3) reaction system may be represented by the following equation:
a CH2xe2x95x90CHxe2x80x94Rxe2x80x94CHxe2x95x90CH2+b(H)3xe2x80x94Sixe2x80x94Rxe2x80x2xe2x86x92xe2x80x94[(CH2)2xe2x80x94Rxe2x80x94(CH2)2xe2x80x94Sixe2x80x94(Rxe2x80x2)]n less than 
where the end-groups of the resulting polymer may be either CHxe2x95x90CH2 or Sixe2x80x94H depending on the value of the a/b ratio used. In addition to vinyl-functionalized monomers, other monomers containing xe2x80x94CH2xe2x80x94CHxe2x95x90CH2, xe2x80x94Cxe2x89xa1CH, xe2x80x94CHxe2x95x90CHxe2x80x94 or other corresponding unsaturated groups could also be used, while both R and Rxe2x80x2 may or may not contain silicon. Also, other combinations of di- and/or polyfunctional (e.g., tri-, tetra, etc.) monomers having functional groups of the same type (e.g., hydrosilyl, vinyl, allyl, etc.) may be used. Some specific examples of applicable silicon-containing unsaturated monomers include diallyldimethylsilane, diallyldiphenylsilane, divinyldichlorosilane, divinyldimethylsilane, divinyltetramethyldisilane, 1,4-divinyltetramethyldisilylethylene, trivinylchlorosilane, trivinylethoxysilane, trivinylmethoxysilane, trivinylmethylsilane, tetraallylsilane, tetravinylsilane, various arylenedivinylsilanes, such as p- or m-phenylenetetraalkyldivinylsilanes, p- or m-phenylenetetraalkyldiallylsilanes, etc. Some examples of corresponding non-silicon-containing monomers include diallyl ether, diallyl maleate, diallyl phtalate, diallyl dicarbonate, diallyl succinate, divinylbenzene, triallyl benzenetricarboxylate, trivinylcyclohexane, etc. If monomers such as 1,3-divinyl-1,3-diphenyl-1,3-dimethyldisilazene,1,3-divinyltetramethyldisilazene, 1,3,5-trivinyl-1,3,5-trimethylcyclotrisilazene, tris(dimethylvinylsilyl)amine, or alike are used, corresponding polycarbosilazenes will be obtained. Some examples of silane monomers include dimethylsilane, diethylsilane, diphenylsilane, phenylmethylsilane, methylsilane, phenylsilane, 1,3-disilabutane (i.e., 1-methyldisilmethylene), 1,1,3,3-tetramethyldisilazene, 1,1,4,4-tetramethyldisilethylene, etc.
In accordance with this invention, a hyperbranched polycarbosiloxane is synthesized by a hydrosilation reaction of compounds having two or more vinyl, allyl or other homologous functional groups with a dihydrido- or polyhydrido-silane or siloxane, wherein at least one of the monomers includes at least three functional groups. Examples of such (A2+B3) or (A2+B4) reaction systems may be represented by the following equations: 
Some specific examples of applicable siloxane monomers include 1,3-diallyltetrakis(trimethylsiloxy)disiloxane, 1,3-diallyltetramethyldisiloxane, 1,3-divinyl-1,3-dimethyl-1,3-dichlorodisiloxane, 1,3-divinyl-1,3-diphenyl-1,3-dimethyldisiloxane, 1,5-divinyl-3,3-diphenyltetramethyltrisiloxane, 1,5-divinylhexamethyltrisiloxane, 1,5-divinyl-3-phenylpentamethyltrisiloxane, 1,3-divinyltetraethoxydisiloxane, divinyltetrakis(trimethylsiloxy)disiloxane, 1,3-divinyltetramethyldisiloxane, divinyltetraphenyldisiloxane, tris(vinyldimethylsiloxy)methylsilane, tris(vinyldimethylsiloxy)phenylsilane, 1,3,5-trivinyl-1,1,3,5,5-pentamethyltrisiloxane, 1,3,5-trivnyl-1,3,5-trimethylcyclotrisiloxane, 1,1,3,3-tetramethyldisiloxane, methyltris(dimethylsiloxy)silane, phenyltris(dimethylsiloxy)silane, methylhydrocyclosiloxanes, tetrakis(dimethylsiloxy)silane, etc. Combinations of these monomers and monomers listed above for the preparation of polycarbosilanes and/or polycarbosilazenes will lead to formation of a variety of different block- or segmented-poly(carbosilane-carbosiloxane), poly(carbosilane-carbosilazene) and/or poly(carbosilazene-carbosiloxane) copolymers.
Hyperbranched polymers are polymers having branches upon branches. More specifically, a hyperbranched polymer contains a mixture of linearly and fully branched repeating units, whereas an ideal dendrimer contains only fully branched repeating units, without any linearly repeating units, and ideal linear polymers contain only linear repeating units, without any branched repeating units. The degree of branching (DB), which reflects the fraction of branching sites relative to a perfectly branched system (i.e., an ideal dendrimer), for a hyperbranched polymer is greater than zero and less than 1, with typical values being from about 0.25 to 0.45.
The average degree of branching ({overscore (DB)}) is defined as the number average fraction of branching groups per molecule, i.e., the ratio of terminal groups plus branched groups to the total number of terminal groups, branched groups, and linear groups. For ideal dendrons and dendrimers the degree of branching is 1. For ideal linear polymers the degree of branching is 0. The degree of branching is expressed mathematically as follows:       DB    _    =                    N        t            +              N        b                            N        t            +              N        b            +              N        l            
where Nt represents the number of terminal units, Nb represents the number of branched units, and Nl represents the number of linear units.
Unlike ideal dendrimers which have a polydispersity near 1, hyperbranched polymers have a polydispersity that increases with increasing molecular weight, with typical polydispersities being greater than 1.1 even at a relatively low molecular weight such as 1,000 Daltons, and with polydispersities greater than 2 being typical for hyperbranched polymers having a molecular weight of about 10,000 Daltons or higher. These differences between the polydispersities and degree of branching of hyperbranched polymers verses dendrimers is indicative of the relatively higher randomness and irregularity of hyperbranched polymers as compared with dendrimers, and distinguishes hyperbranched polymers from dendrimers.
An important aspect of this invention relates to control of the polymerization process to prevent gelation (i.e., cross-linking) of the copolymer. For the copolymerization processes of this invention, the ratio (r) of A-groups to B-groups (r=A/B), and/or the extent of reaction (p) are selected so as to avoid gelation. A hyperbranched polymerization system can be approximately modeled by assuming that (a) A-groups only react with B-groups, and B-groups only react with A-groups, (b) cyclization reactions do not occur, (c) all A-groups exhibit equal reactivity regardless of the size and shape of the molecule to which they are attached, and (d) all B-groups exhibit equal reactivity regardless of the size and shape of the molecule to which they are attached. Using these assumptions for the general system Ax+By, it can be theoretically determined that complete conversion of the minor component can be achieved without gelation if 1/[(xxe2x88x921)(yxe2x88x921)]xe2x89xa7rxe2x89xa7(xxe2x88x921)(yxe2x88x921). As an example, complete conversion of the minor component can be achieved for an A2+B3 system without gelation when the ratio of A-groups to B-groups is less than 0.5 or greater than 2. As an alternative, gelation can be avoided by controlling the extent of conversion (completion) of the reaction. As an example, for an Ax+By system, it can be theoretically determined that gelation can be avoided when r=1 (i.e., the number or concentration of A-groups is equal to the number or concentration of B-groups) if the extent of reaction (p) is less than [1/(xxe2x88x921)(yxe2x88x921)]xc2xd. For example, for an A2+B3 system in which the number of A and B groups are equal, gelation can be avoided if the reaction is terminated at or below about 70% completion. Methods of quenching, or otherwise stopping a polymerization reaction at a desired extent of conversion are known and will not be described in detail.
Generally, gelation can be avoided by selecting the extent of reaction (p), the ratio of A-groups to B-groups (r), the number of A-functional groups per molecule of A-functional monomer (x), and the number of B-functional groups per molecule of B-functional monomer (y) so that the relation rp2xe2x89xa61/[(xxe2x88x921)(yxe2x88x921)]. In the case where either A-groups or B-groups are in excess (i.e., rxe2x89xa01), the extent of reaction (p) is determined with respect to the minor reactant (the lesser of A and B).
Because the above relationships are dependent on assumptions that only approximate real systems, the actual degree of conversion that can be achieved for a given ratio of A-groups to B-groups before gelation occurs tends to be slightly higher than the theoretically predicted conversion. However, the theoretically determined conversion is an excellent starting point from which to conduct experiments to determine the actual allowable extent of conversion before gelation occurs. Similarly, the allowed ranges for the ratio of A-groups to B-groups for a given extent of conversion without the occurrence of gelation for a real system may be somewhat broader than the theoretically predicted ranges. However, the theoretically predicted ranges provide an excellent starting point for a series of experiments to determine how close to r=1 a system can be before gelation occurs.
Suitable reaction conditions, such as temperature, pressure and solvents, and suitable catalysts are the same as those used for conventional hydrosilation reactions. These conditions and catalysts are well known and will not be described in detail.
The polymerization processes of this invention may be performed using bulk or solution polymerization techniques. The monomers may be added to a reaction vessel either together or separately, and may be added all at once, incrementally, or continuously. The copolymerization reactions of this invention are preferably achieved as a batch process in a single reaction vessel. However, it is contemplated that the processes of this invention may be amenable to continuous reaction processes, such as continuous stirred tank reactors and plug flow reactors. However, the processes of this invention do not encompass an iterative sequence of reactions and separations of the type associated with the synthesis of dendrimers.
The hyperbranched polymers prepared by the processes of this invention will typically have a degree of branching of less than 55%, and more typically from about 25% to about 45%. Their typical polydispersities may range from about 1.1 for lower molecular weight hyperbranched polymers (e.g., those having a molecular weight of about 1,000 Daltons) to 2 or higher for higher molecular weight hyperbranched polymers (e.g. those having a molecular weight of 10,000 Daltons or more). The hyperbranched polymers that are prepared in accordance with the processes of this invention typically have a weight average molecular weight of about 25,000 Daltons or less. However, higher molecular weight hyperbranched polymers can be prepared. The typical molecular weights for the hyperbranched polymers prepared in accordance with this invention are from about 1,000 (weight average) to about 25,000 (weight average), and more typically from about 1,000 to about 10,000 Daltons (weight average).
The hyperbranched polycarbosilanes, polycarbosiloxanes, polycarbosilazenes and copolymers thereof of this invention may be prepared by combining a di- or poly-, vinyl or allyl-functionalized monomer with a di- or polyhydrido-functionalized monomer under suitable conditions for effecting polymerization via hydrosilation reactions, with the number of vinyl and/or allyl groups exceeding the number of hydrido groups to provide a vinyl or allyl terminated hyperbranched polymer or with the number of hydrido groups exceeding the number of vinyl or allyl groups to provide a hydrido-terminated hyperbranched polymer.
The surface functional groups (i.e., terminal groups) of the hyperbranched polycarbosilanes, polycarbosiloxanes, polycarbosilazenes and copolymers thereof of this invention can be chemically modified to provide generally any desired surface functionality. Surface functionality can be modified to facilitate subsequent reactions and/or to impart desired solubility or other application properties, such as sensory, catalytic, etc. For example, a hydrido-terminated hyperbranched polycarbosilane may be reacted with a vinyl or allyl-functional silane or siloxane having one or more hydrolyzable groups (e.g, methoxy- or ethoxy-groups) bonded to a silicon atom to form a hyperbranched polycarbosilane that can be cured (cross-linked) via hydrolysis/condensation reactions upon exposure to moisture.
In general, the hyperbranched polycarbosilanes, polycarbosiloxanes, polycarbosilazenes and copolymers thereof of this invention may be covalently connected to each other to form a nano-domain-structured network using alpha,omega-telechelic linear polymers or oligomers, multi-functional linear polymers with functional groups pendant to the main chain backbone, and/or multi-functional randomly branched polymers having functional groups regularly or randomly distributed in the main or in the side chains. Other types of connectors may include di- or multi-functional low molecular weight compounds that can react with hyperbranched polymer end groups. Connectors may also include multi-arm star polymers, dendrimers, dendrons, homologously derivatized or other hyperbranched polymers, or other architecturally specific macromolecules. Nano-domain networks formed from the hyperbranched polycarbosilanes, polycarbosiloxanes, polycarbosilazenes and copolymers thereof of this invention may be viewed as three-dimensional, cross-linked materials comprising covalently bonded nanoscopic, hyperbranched domains which may be of the same or different chemical composition than the rest of the network. These materials may be formed into clear, transparent films, sheets, membranes, coatings or other objects, and may exhibit glass transition temperatures that may rank them among either elastomers or plastomers. The materials may also exhibit high thermal stability, mechanical strength and toughness, and may offer new ways of preparing specialty membranes, protective coatings, photoresists, novel composites, controlled porosity materials, etc. Other applications may be found in biomedical areas, medical science and engineering, purification of liquids and gases, food processing, storage and packaging, printing and lithography, sensors, catalysts, etc. In many applications, such as coatings, the hyperbranched polymers may exhibit lower viscosity at any given solids content as compared with conventional compositions using linear or lightly branched polymers, and in contrast to dendrimers may exhibit desirable shear-thinning properties for certain applications.